Conventional seating furniture is designed in most cases so that the body, especially the back, is supported by correspondingly fashioned seating surfaces and backs in an anatomically maximally favorable position. Although such seating furniture is frequently felt to be comfortable, there is the decisive drawback that the body sits merely passively on such seats, i.e. the back muscles are hardly stressed, and the inter-vertebral disks are stressed merely statically in the "pressure mode". As a result, a long-term usage of such seat furnishings leads to degeneration of the back muscles and wasting of the intervertebral disks. Impairment of health and pains in the back and hip regions (e.g. sciatica) are the frequent consequence of such static and passive sitting.
For this reason, seating furnishings have been developed permitting a so-called active dynamic sitting wherein the back musculature and the intervertebral disks are constantly slightly active. This active dynamic sitting attitude is attained in practically all cases by maintaining the actual seat of the seating furniture in a labile position and making it optionally additionally resilient in the vertical direction.
Such an active dynamic seating device has been described, for example, in DE 73 11 140. This seat consists essentially of a seat part connected via a first tilting joint with a supporting shank, the latter, in turn, being articulated by means of a second tilting joint to the base of the seating device. In this arrangement, each tilting joint consists preferably of a cap formed respectively at the end of the supporting shank, this cap being guided in a hollow cylinder and stressed by a coil spring arranged in the hollow cylinder.
On account of the planar structure of the underside of the cap, the latter is in contact, in the non-stressed condition, with the bottom or, respectively, top of the hollow cylinder so that, without stress, a perfect alignment is achieved of base, supporting shank, and seat. When stress is exerted on this seating device, the two coil springs of the tilting joints are compressed, the two caps being urged into the two hollow cylinders. The tilting movement of these two joints is attained by the feature that the bore in the top of the lower cylinder or, respectively, in the bottom of the upper hollow cylinder is slightly larger than the outer diameter of the supporting shank.
However, the disadvantage arises herein that the maximally possible tilting angle of each tilting joint in the stressed condition is dependent on the distance of the planar side of the cap from the bottom or top of the hollow cylinder and thus on the weight of the person presently using this seating device. Moreover, it is extremely difficult to maintain one's balance on this seating device so that, at least for inexperienced users, there must be the possibility that at least one of the tilting joints is blocked. This results from the fact that, upon deflection of the tilting joint at the base of the seating device into a specific direction, a deflection of the upper tilting joint in the same direction takes place in a preferred manner.